Unit injector employing hydraulically controlled timing and fuel shut off

ABSTRACT

A unit injector (200,300,400,500) of the cam actuated, open nozzle type is disclosed including an extremely compact injector body (202,302,402,502) having a minimum number of internal flow passages and a two piece injector plunger (222,308,310,412,506,510) arranged to provide pressure/time metering (all embodiments), scavenging fuel flow (all embodiments), hydraulic timing (all embodiments), fuel cut off (all embodiments), wear compensation in advance timing mode (all embodiments) and sharp cut off of fuel injection without high &#34;crush&#34; loads and wear injection without high &#34;crush&#34; loads and wear compensation in retard and advance modes (embodiment of FIG. 3). All embodiments of this invention are characterized by an injector body (202,302,402,502) having an exterior shape designed to form a fuel supply flow path and an isolated fuel drain flow path between the exterior surface of the injector body (202,302,402,502) and the interior surface of the recess in which the injector (200,300,400,500) is designed to be received. This arrangement allows a very simple pattern of passages (mostly radial passages) to be formed in the injector body.

DESCRIPTION

1. Technical Field

This invention relates to a miniaturized, variable timing fuel injector having a two piece injector plunger mechanically actuated by an engine camshaft.

2. Background Art

Internal combustion engine designers have increasingly come to realize that substantially improved fuel supply systems are required in order to obtain higher levels of pollution abatement and increased fuel economy. Among the known options, direct fuel injection appears to be one of the best candidates for achieving improved performance but higher initial costs have tended to discourage its general adoption. This situation is accentuated by the fact that only the more sophisticated, and therefore normally the more expensive, direct injection systems are capable of achieving the increasingly higher performance goals of engine manufacturers.

Up to the present, attempts to provide a low cost fuel injection system have tended to center on distributor type fuel injection systems having a single centralized high pressure pump and a distributor valve for metering and timing fuel flow from the pump to each of a plurality of injection nozzles, such as disclosed in U.S. Pat. No. 3,557,765. Although simple in design concept, systems of this type generally suffer defects inherent with separation of the injector nozzles from the centralized pump. Unit injector systems avoid the inherent defects of distributor type systems by providing each engine cylinder with its own cam-actuated pump such as disclosed in U.S. Pat. No. 3,544,008. Nevertheless, the performance advantage of unit injectors have generally not outweighed the detriment of greater costs except for heavy duty compression ignition engine applications.

While unit injectors have not normally been employed in low cost fuel systems, numerous refinements have been proposed over the years in an attempt to lessen their cost while retaining their inherent advantages. For example, the assignee of this application, Cummins Engine Company, Inc., has developed an open nozzle, pressure/time unit injector (see U.S. Pat. Nos. 3,351,288 and 3,544,008) which is characterized by the need for only a single supply line (common rail) for supplying fuel to all of the injectors. Because fuel is metered into each injector through a separate feed orifice, the time during which each feed orifice is open and the pressure within the common rail can be relied upon to control the quantity of fuel metered for injection during each injection cycle. Additional cost reductions are realized because an open nozzle unit injector does not require a pressure relief valve at the injection orifice.

While effective in providing cost reductions, certain precautions are usually required in pressure/time, open nozzle injector systems to insure that combustion gases and spurious pressure signals do not enter the fuel supply system. These precautions may include the use of a check valve, upstream from the feed orifice of the injector to discourage combustion gases (known as blow-back) from entering the fuel supply. It is also common to provide for scavenging flow of fuel through the injector to remove gases which may have entered the fuel supply line. Scavenging flow additionally serves the function of cooling the injector and would be useful even if it were unnecessary to remove blow-back gases. Thus, a commercially acceptable unit injector, even when designed in accordance with principles intended to simplify the injector structure, is typically a labyrinth of axial and radial drillings. As used in this application, "axial" means parallel with the longitudinal axis of the injector and "radial" means perpendicular to the longitudinal axis of the injector. Moreover, the injector body is usually formed of multiple components requiring very close tolerance controls.

The problem of complexity is further magnified when it is desired to provide a cam operated unit injector with the capability of advancing and/or retarding fuel injection during each injector cycle. U.S. Pat. Nos. 4,249,499 and 3,951,117 provide examples of pressure/time unit injectors which respond to a hydraulic variable pressure signal to control injector timing. Both of these patents disclose unit injectors in which a variable length hydraulic link is inserted into the mechanical actuating train between the engine cam shaft and the injector plunger which reciprocates to force fuel through the injector nozzle. FIGS. 16 and 17 of U.S. Pat. No. 3,951,117 specifically disclose a cam actuated, pressure/time type, open nozzle injector wherein the injector plunger is formed in two sections to create a timing chamber between them. During the metering portion of the injector cycle, fuel is metered into the timing chamber in an amount corresponding to the pressure of the fuel supplied thereto. Numerous other examples of cam actuated unit injectors employing hydraulically variable timing are known such as disclosed in U.K. Pat. No. 1,080,311 and U.S. Pat. Nos. 2,863,438, 3,847,510 and 3,859,973. Hydraulic timing control is also known in other types of fuel injectors such as disclosed in U.S. Pat. No. 3,083,912 wherein a reciprocating piston is subjected to hydraulic pressure against the bias of a coil spring to control injector timing. In addition to purely hydraulic control, mechanical and hybrid mechanical/hydraulic variable timing controls have been applied to cam actuated unit injectors as illustrated in U.S. Pat. Nos. 2,313,264 and 2,997,994. Such mechanical systems tend, however, to increase the overall size and cost of the fuel injection system to say nothing of the possibility for mechanical malfunctions. If other injector capabilities are desired, such as the capability to cut off fuel flow entirely (see U.S. Pat. No. 2,984,230), the structure of the injector will normally become still more complex.

In very simple cam actuated injectors such as illustrated in Australian Pat. No. 202,734, axial drillings have been eliminated from the injector body by forming a flow path between the exterior of the injector body and the interior of the recess in which the injector body is received. By this arrangement, an easily formed radial bore may be provided in the injector body to act as a feed passage from the supply flow path surrounding the injector body into the injector chamber of the unit injector. If the injector is capable of providing a scavenging fuel flow as illustrated in U.S. Pat. No. 3,351,288, the number of necessary flow passages increases and the overall size of the injector must increase to accommodate such passages. Multi-function injectors of this type normally require at least one or more axial passages. Such passages are more difficult and costly to form and have not been eliminated even in injector designs in which the supply and drain lines leading to and from the injector are formed entirely within the engine head such as is illustrated in U.S. Pat. No. 3,351,288.

In short, fuel injectors capable of meeting rigorous operation specifications have invariably been highly complex, costly to manufacture, and/or relatively large in size.

DISCLOSURE OF THE INVENTION

The first object of this invention is to overcome the deficiencies of the prior art as disclosed above by providing a unit fuel injector having a two piece reciprocating plunger which may be operated in retard, advance or fuel cut off modes by simply varying the pressure of fluid supplied to a chamber formed between the plunger sections.

The second object of this invention is to provide a unit fuel injector including a two piece, cam-actuated plunger in which the plunger sections are biased together by a force which varies in steps as a function of the axial separation of the plunger sections.

A third object of this invention is to provide a unit injector having a simplified arrangement of flow passages providing hydraulic timing, scavenging and fuel cut off functions in combination with a two piece, reciprocating plunger in which the plunger sections are biased together by at least one extension spring secured to one of the plunger sections by a lost motion connection which permits a predetermined amount of motion between the plunger sections before the extension spring becomes operative to urge the plunger sections together.

The fourth object of the subject invention is to provide a highly compact unit injector designed to perform metering, scavenging and timing functions by means of an extremely simple injector body adapted to form isolated supply, drain and timing flow paths between its exterior surface and the interior surface of the recess within which the injector is received, wherein the injector includes an injector plunger formed in two pieces biased together by a pair of oppositely wound spring elements to form a variable volume timing chamber in which the biasing means includes a first spring integrally connected at its ends to the plunger sections and a second, stronger spring integrally connected directly with one plunger section and to the remaining plunger section through a lost motion connection. By this arrangement, the effective length of the injector plunger may be adjusted by timing fluid pressure variations to advance or retard the injection on a cycle-by-cycle basis or to hold the inner plunger section in its innermost position to cause total cut-off of fuel flow from the injector nozzle.

The fifth object of this invention is to provide a highly compact unit injector containing a hydraulic timing chamber formed between inner and outer plunger sections biased together by a pair of extension springs with the first spring being connected to one of the plunger sections through a first lost motion connection means and the second spring being connected through a second lost motion connection means attached to the first lost motion connection means whereby a minimal hydraulic link, defined by the first lost motion connection means, is always formed and collapsed at the end of each injection stroke of the injector plunger and a longer length hydraulic link, defined by the second lost motion connection means, is formed to advance timing upon an increase of pressure in the timing fluid supplied to the injector and whereby upon a still greater increase in timing fluid pressure, the inner plunger section may be held in its innermost position throughout successive injector cycles to cut off fuel flow entirely.

A sixth object of this invention is to provide a unit injector of simplified design including a two piece, injector plunger between which is formed a variable length timing chamber which expands and collapses during each injection cycle and including means for applying a biasing force to the plunger sections which is a stepped function of the effective length of the timing chamber. By providing a spill restriction during collapse of the variable length timing chamber, a sharp end of injection and automatic wear compensation are achieved in all modes of injector operation.

Still other and more specific objects of this invention may be understood from an examination of the following Brief Description of the Drawings and Best Mode for Carrying Out the Invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially broken away, cross-sectional view of an unit injector designed in accordance with the subject invention in which the injector provides both variable timing and a sharp fuel shut off at the end of each injector function.

FIGS. 2a-2f are cross-sectional views of the unit injector of FIG. 1 operating in different phases.

FIG. 3 is a cross-sectional view of another embodiment of the subject invention in which a wear compensation function and a sharp end of injection function during the retard mode of operation have been added to the injector design of FIG. 1.

FIG. 3a is a chart which illustrates how the injector embodiment of FIG. 3 is capable of achieving a sharper end of injection.

FIG. 4 discloses yet another embodiment of an unit injector designed in accordance with the subject invention to provide both variable timing and fuel cut off capabilities wherein the timing and drain passages have been reversed axially from the configuration illustrated in FIGS. 1 and 3.

FIG. 5 is a split cross-sectional view of still another embodiment of a unit injector designed in accordance with the subject invention wherein the injector plunger sections are biased together by a pair of compression springs.

FIG. 6 illustrates a split cross-sectional view of the unit injector of FIG. 5 in which the injector plunger is shown in two alternative conditions.

FIG. 7 is a graph which illustrates plunger separation distance (lash) as a function of timing fluid pressure.

BEST MODE FOR CARRYING OUT THE INVENTION

Throughout this application, the words "inward" and "outward" will correspond to the directions, respectively, toward and away from the point at which fuel from an injector is actually injected into the combustion chamber of an engine. To further aid in an understanding of this invention, an index of the elements identified by reference numerals in the drawings is listed at the end of this description of the Best Mode for Carrying Out the Invention.

FIG. 1 illustrates an open nozzle, pressure/time unit injector designed in accordance with the subject invention. In particular, FIG. 1 shows a unit injector 200 including an injector body 202 received in a recess 203 contained in the head 205 (only partially illustrated) of an internal combustion engine. Injector body 202 is actually formed in two sections including an outer section or barrel 202a and inner section or cup 202b which are integrally joined together. As is apparent from FIG. 1, injector body 202 is held within the recess 203 by means of mating screw threads 202c and may be placed in position by means of a wrench adapted to mate with a hexagonal socket 203a contained in the outer end of the injector body 202. Fuel supply and drainage is provided for the injector through passages contained entirely within head 205. In particular, supply channel 204 (sometimes referred to as a common rail) is adapted to supply fuel to the unit injector 200 while drain channel 206 is adapted to receive fuel discharged from the injector for return to the fuel supply system. Both supply channel 204 and drain channel 206 may be formed to communicate with each of a plurality of injectors positioned within head 205 to eliminate all external flow passages to and from the injectors contained in the engine head. In addition to the fuel supply channel 204 and fuel drain channel 206, which communicate with recess 203 through supply port 208 and drain port 210, respectively, an additional timing channel 212 is formed in the engine head intermediate the drain and supply channels to provide timing fluid to each of the injectors within the engine head. This timing channel 212 communicates with recess 203 through a timing port 214.

The exterior surface of injector body 202 is shaped to provide three annular recesses 236, 234 and 238 axially spaced to communicate with channels 204, 212 and 206 through supply ports 208, timing port 214 and drain port 210, respectively. By this arrangement, a supply flow path 216 is formed between supply port 208 and the exterior surface of the injector body 202 defining recess 236. Similarly, a drain flow 240 is formed between drain port 210 and the exterior surface of the injector body 202 which defines annular recess 238. A timing fluid flow path 232 is formed by annular recess 234 placed between supply flow path 216 and drain flow path 240. In order to isolate supply flow path 216 from timing flow path 232, a circumferential seal 241 is positioned between injector body 202 and the interior surface of recess 203. As is apparent from FIG. 1, the circumferential seal 241 is positioned within a small annular groove 241a located between annular recesses 234 and 236. A similar type of circumferential seal 243 is positioned within a second small annular groove 243a located between annular recesses 234 and 238 to insure against fuel leakage between the timing flow path 232 and the drain flow path 240. The inner end of supply flow path 216 is sealed by means of a washer-like seal element 245 compressibly held between an inwardly directed lip 247 formed in recess 203 and a radially outwardly directed surface 249 formed on injector body 202. An outer circumferential seal 253 received in small annular groove 253a on the injector body 202 above (as illustrated in FIG. 1) annular recess 238 serves to seal the drain flow path 240 against outward leakage of fuel.

Injector body 202 contains a central bore 228 which is coaxial with the longitudinal axis of the injector body 202. At the inner (lower) end of the injector body 202, one or more small injection orifices 251 are contained to provide a communication path between central bore 228 and a combustion chamber. Fuel is supplied through injection orifices 251 to the combustion chamber by the injector in controlled synchronism with the operation of the corresponding cylinder piston (not illustrated). Positioned within central bore 228 is an injector plunger 222 adapted to reciprocate in response to a cam-actuated mechanism 270 including a specially profiled cam lobe 271, connected to the conventional cam shaft (not illustrated) of the engine, a push rod 272, rocker arm 273 and a link 274. Because cam lobe 271 rotates in a fixed manner relative to the reciprocating motion of the cylinder piston, plunger 222 may have its reciprocating motion very pecisely controlled with respect to the cylinder piston by carefully shaping the profile of cam lobe 271. Cam mechanism 270 may be of the type which employs a stepped cam to hold, during a sustained portion of each injection cycle just prior to injection, the injector plunger in an inward position in which the feed orifice of the injector is closed to prevent combustion gas blow back. A cam-actuated mechanism of this type is disclosed in the commonly assigned application of Les Peters entitled Fuel Injector Assembly Including a Blow-Back Prevention Cam Ser. No. 336,334, filed Dec. 31, 1981. The entire disclosure of that application is incorporated by reference.

Essentially, injector plunger 222 reciprocates between an innermost position in which injection orifices 251 are closed by the forward end 223 of plunger 222 and an outermost position in which an injection chamber is formed within central bore 228 between the forward end 223 and the injection orifices 251. Since this invention relates to injectors of the "open nozzle" type, orifices 251 remain open to the combustion chamber whenever injector plunger 222 is retracted from its innermost position. Injectors of this type have an inherent cost advantage over more complex closed nozzle injectors which employ a pressure released tip valve for maintaining the injection orifice in a closed condition except during the injection event. Injection plunger 222 is permanently biased toward its outermost position by a plunger return means 275 including a sleeve like coupler 276 connected at its lower end 277 to the upper end of injection plunger 222. A radially, outwardly directed flange 278 is positioned adjacent the topmost portion of coupler 276. A fairly high pressure compression spring 279 extends between the upper end of injector body 202 and flange 278 to continually bias plunger 222 in the outer direction.

To control the amount of fuel injected during each forward movement of injector plunger 222, the subject injector is provided with a "pressure/time" injection capability in which fuel is metered into the injection chamber before each injection stroke through a fill or feed orifice which has been carefully dimensioned to allow the amount of fuel injected to be varied within a given amount of time by varying the supply line (common rail) pressure. Thus, the amount of fuel actually metered is a function of the supply pressure and the total metering time during which fuel flows through the feed orifice. This principle is applied to the subject unit injector by provision of a supply connecting means 217 for providing a pathway for fuel under pressure to flow from the supply flow path 216 into a section of the central bore 222 which forms the injection chamber. The supply connecting means 217 includes a supply passage 218 extending from the exterior surface of the injector body 202 to the interior surface of the central bore 222 along a straight radial path perpendicular to the longitudinal axis of the injector body 202. A restricted portion of supply passage 218 forms a feed orifice 219 which has carefully controlled hydraulic characteristics in order to produce the desired pressure/time metering capability discussed above.

The amount of time which the injector plunger 222 dwells in either the innermost or outermost position depends in part upon the profile of cam lobe 271. As is apparent from FIG. 1, a preferred design for cam lobe 271 includes one sector 280 comprising more than 180° of the total circumference. This sector causes the injector plunger to assume its innermost position. A second sector 281, comprising almost 90° of the total circumference, causes plunger 222 to assume its outermost position.

Separating first sector 280 and second sector 281 is a third sector 282 which functions to move the injector plunger 222 to a supply passage blocking position in which the injector plunger 222 is moved inwardly sufficiently to close off feed orifice 219 but does not move the injector plunger 222 sufficiently to actually initiate fuel injection. Third sector 282 is shaped to cause feed orifice 219 to be closed during the time that combustion chamber pressures are rising to a sufficiently high level to cause significant blow back of combustion gases into the fuel supply line. A fourth small sector 283 of the cam lobe 279 represents approximately that portion of the cam lobe 271 which causes actual fuel injection. The point at which injection begins is approximate since it depends in part upon the amount of fuel metered into the injection chamber during each cycle.

First sector 280 engages the lower end of push rod 272 at the point just before the engine piston executes its power stroke and continues to engage push rod 272 throughout the exhaust stroke and a portion of the intake stroke of the piston. Second sector 281 engages rod 272 approximately halfway through the intake stroke of the piston and ceases to engage rod 272 approximately halfway into the compression stroke. Second sector 281 defines the time during each injector cycle when fuel is metered into the injection chamber. Third sector 282, which causes the injector plunger 222 to move toward its innermost position sufficiently to close the feed orifice 219 but not to cause injection, is positioned to engage push rod 272 approximately halfway through the compression stroke and to continue this engagement up to the point at which injection commences. The fourth small sector 283 is that portion of cam lobe 271 which causes injector plunger 222 to move from its outermost position to its innermost position to force the metered fuel in the injection chamber through injection orifices 251 into the combustion chamber of the engine. A slightly raised dimple 284 exaggerated in FIG. 1 is formed on cam lobe 271 near the end of fourth sector 283 to place a "crush load" on injector plunger 222 at the end of the injection event in order to hold the injector plunger 222 tightly against the injector cup 202b. This ability to hold injector plunger 222 very tightly against the injector cup 202b insures that injection orifices 251 will be sharply closed and will remain closed at the end of each injection period even during the retard mode of operation of the injector embodiment of FIG. 1. An essential characteristic of the injector of FIG. 1 is that it allows the fuel injection event to be advanced relative to the movement of the associated engine piston by the simple expedient of varying the pressure of a timing fluid supplied through the timing fluid channel 212. Since all of the engine injectors are in communication with the same timing channel, a very simple control may be provided to change the timing of all of the engine injectors.

To permit this adjustment in timing, the injector plunger 222 of each injector is formed in two sections including an inner plunger section 224 and an outer plunger section 226. The two sections are independently mounted within the central bore 228 of the injector body 202 to form a variable volume timing chamber 230 (illustrated in its fully collapsed condition). Timing fluid is supplied to chamber 230 by the timing fluid flow path 232.

Timing fluid flows into the central bore 228 through a timing fluid connecting means 242 which provides a pathway for fuel flow from the timing fluid flow path 232 into the central bore 228. The timing fluid connecting means 242 includes a timing passage 244 extending from the exterior surface of the injector body 202 to the interior surface of the central bore 228 along a straight radial path perpendicular to the longitudinal axis of the injector body 202. The exterior surface of inner plunger section 224 is shaped to form a flow control means 246 for controlling the flow of fuel and timing fluid within the injector. In particular, the flow control means 246 includes an annular groove 248 axially positioned to communicate at all times with timing passage 244. Flow control means 246 further includes a transverse passage 250 in the inner portion of annular groove 248 to communicate at its ends with the space formed by annular groove 248 within central bore 228. The outer end of inner plunger section 224 contains a recess 252 at the bottom of which is a ball-type check valve 254 designed to allow one way fluid flow from the transverse passage 250 into the recess 252. It should be apparent from FIG. 1 that recess 252 communicates directly with the variable volume timing chamber 230 in a manner to permit the flow of timing fluid through timing passage 244, annular groove 248, transverse passage 250, check valve 254 and recess 252. Flow control means 246 on inner plunger section 224 also includes a cylindrical land 248a which sealingly engages the interior surface of central bore 228 to seal the space within central bore 228 defined by annular groove 248 from the injection chamber of the injector. As illustrated in FIG. 1, cylindrical land 248a is axially positioned to open and close feed orifice 219 in the manner described above.

Biasing means 256 is provided for continuously biasing the inner and outer plunger sections together. Biasing means 256 includes first helical extension spring 258 connected integrally at each end, respectively, to the plunger sections to apply at all times at least a first predetermined force tending to bring the plunger sections together. A second helical extension spring 260 mounted coaxially within the first extension spring 258 is connected at one end to inner plunger section 224 and at the other end to outer plunger section 226 by a lost motion connection means 262 to cause the second extension spring to apply a second predetermined force tending to bring the plunger sections together whenever the plunger sections are separated by more than a predetermined distance defined by the lost motion connection means 262. An illustrated in FIG. 1, lost motion connection means 262 is positioned with an inwardly opening recess 226a formed in outer plunger section 226. The lost motion connection means 262 includes a pin 262a extending through recess 226a generally perpendicular to the longitudinal axis of the outer plunger section 226. The ends of pin 262a are fixed in the same manner, such as by press fitting, to the adjacent portion of outer plunger section 226. Held within recess 226a by pin 262a is an anchor element 262b containing an elongated opening 262c which receives pin 262a. Obviously, the axial length of opening 262c defines the relative movement which may take place between outer plunger 226 and anchor element 262b when the injector plunger sections move from the position illustrated in FIG. 1 to a position in which second helical spring 260 will begin to apply a force on the plunger sections tending to pull the sections together. It is thus the axial length of opening 262c which defines the "lost motion" which may occur between the plunger sections before spring 260 becomes effective. The ends of extension spring 258 are connected by means of helical screw threads 258a and 258b to inner and outer plunger sections 224 and 226 respectively. Similarly, the ends of spring 260 are connected by screw threads 260a and 260b to the inner plunger section 224 and anchor element 262b. To prevent relative rotation of the plunger sections, the springs 258 and 260 are wound in opposite directions.

The unit injector illustrated in FIG. 1 includes a drain connecting means 264 for providing a pathway for fuel flow from the central bore 228 into the drain flow path 240 and a leakage connecting means 265 for providing a pathway for fuel leaked outwardly from the timing chamber 230 to flow into the drain flow path 240. The leakage connection means 265 includes an annular cavity 267 extending radially outwardly from the central bore 228 and a leakage passage 269 extending from the annular cavity 267 to the drain flow path 240. The drain connecting means 264 further includes a drain passage 266 extending obliquely from the central bore 228 to the exterior surface of annular recess 238. The point of communication between drain passage 266 and central bore 228 occurs at the axial location defined by the point of contact between the plunger sections when the inner plunger section 224 is in its innermost position as illustrated in FIG. 1. The innermost end of outer plunger section 226 includes radial passages or grooves 268 (illustrated in dashed lines) which are arranged to provide a fluid communication path for timing fluid to exit from the variable volume timing chamber 230 when the inner plunger section 224 is in its innermost position. Passage 266 includes a restricted portion 266a which operates to control the rate at which fluid is spilled from the timing chamber 230 as will be explained below.

The operation of the unit injector illustrated in FIG. 1 will now be explained with reference to FIGS. 2a-2f. In FIG. 2a, a cam actuating mechanism (not illustrated) has moved the injector plunger 222 to its innermost position in which all fuel metering has been cut off but timing fluid is flowing through the injector to provide one of the normal scavenging functions. In particular, the scavenging flow in the embodiment functions primarily to cool the injector. This function occurs when timing fluid, which may be fuel, enters the timing passage 244 and flows through annular groove 248, transverse passage 250, check valve 254, recess 252 (not illustrated) and grooves 268 to exit through drain passage 266. Because the cam actuating mechanism causes outer plunger section 226 to be held in its innermost position, the timing chamber 230 may not expand from its fully collapsed condition. The scavenging (cooling) flow path is illustrated by arrows 270a.

FIG. 2b shows the phase of injector operation in which the plunger has been moved to its outermost position to open the supply passage 218 and close drain passage 266 to begin to trap timing fluid within the variable volume timing chamber 230. In FIG. 2b both timing and metering have just commenced. Accordingly, very little fuel has been metered into the injection chamber. As disclosed above, metering is in accordance with pressure/time principles. Timing fluid flows into chamber 230 when the injector is in the phase illustrated in FIG. 2c only if the timing fluid pressure is sufficient to overcome the preload tension applied to the plunger sections by extension spring 258. FIG. 2c shows the effect of such a sufficiently high timing pressure having been applied at the moment that the injector plunger commences its inward movement. At this time, check valve 254 operates to trap the timing fuel within the timing chamber 230 to form a hydraulic link between the inner and outer plunger sections. If the timing pressure has not been sufficiently high, no expansion of the timing chamber 230 will have taken place as illustrated in FIG. 2d. It can, thus, easily be seen that the moment of fuel injection may be advanced by expanding the variable volume timing chamber 230 on a cycle by cycle basis. As long as the timing fluid pressure within chamber 230 does not exceed the combined tension force applied by the first and second extension springs 258 and 260, the maximum amount by which the timing chamber can be expanded is limited by the lost motion defined by lost motion connection means 262. While not contemplated for the preferred embodiments of this invention, timing advance could be made infinitely variable by operating the timing on the pressure/time principle. For example, the check valve opening could be carefully sized to limit the amount of fluid entering chamber 230 dependent upon the pressure of the timing fluid and the length of time that the injector is in the phase illustrated in FIG. 2c. Alternatively, the amount of timing advance could be made to depend on the precise pressure of the timing fluid alone independent of the timing period.

In FIG. 2e, the injector is shown in the condition assumed as the plunger reaches its innermost position. In moving from the condition illustrated in FIG. 2c to that illustrated in FIG. 2e, all of the fuel metered into the injection chamber is expelled through the injection orifice as illustrated by arrows 272e and any timing fluid metered into chamber 230 is expelled through the drain passage 266 as illustrated by arrow 274e. Thus, drain passage 266 also forms a timing fluid discharging means for providing a pathway for timing fluid to flow from the timing chamber 230 into the drain flow path 240. Accordingly, drain passage 266 also serves as a discharge passage for timing fluid.

During certain modes of engine operation, such as engine braking in which the engine is used to provide a braking function, it is often desirable to completely cut off all fuel injection. This function may be performed by substantially increasing the timing fluid pressure to a level such that the pressure within the variable timing chamber 230 exceeds the tension force applied by the first and second extension springs 258 and 260 thereby causing the inner plunger section 224 to remain in its innermost position while the outer plunger section 226 reciprocates under operation of the cam actuating mechanism. In this situation, timing fluid will flow into the timing chamber 230 during upward movement of the inner plunger section 224 and will be expelled through drain passage 266 upon downward movement of plunger 222.

It is now apparent that the unit injector embodiment of FIGS. 1 and 2 is capable of performing a variety of injector functions which have heretofore required a much greater number of injector components. In particular, this embodiment provides a pressure/time fuel metering function and a timing function as well as a cooling flow function using the timing fluid as a source of cooling fluid. The injector of FIG. 1 may also be operated to completely cut off fuel injection despite continued engine operation and reciprocation of the cam actuating mechanism.

FIG. 3 illustrates another embodiment of the subject invention in which wear compensation and a sharper end of injection capability (even when the injector timing is not advanced) have been added to the injector design of FIGS. 1 and 2a-2f by the inclusion of a second lost motion connection means. In particular, FIG. 3 illustrates an injector unit 300 including an injector body 302 formed in a manner similar to the injector body 202 of FIG. 1. The supply passage 304 and timing passage 306 are formed to function in the same manner as the corresponding passages in the embodiment of FIG. 1. However, the length of the inner plunger section 308 and outer plunger section 310 have been adjusted to cause the axial location of the variable volume timing chamber 307 to be moved axially outwardly (upwardly) to permit a dual function drain and discharge passage 312 to be formed in the injector body 302 alone a radial path perpendicular to the longitudinal axis of the injector body 302. Leakage passage 314, however, is now formed obliquely with respect to the injector body 302.

A major difference between the embodiment of FIG. 3 versus the embodiment of FIG. 1 is the inclusion of an additional lost motion connection means 316 between the outer (upper end) of the first extension spring 318. By this arrangement, the plunger sections 308 and 310 will always undergo a predetermined separation during each injection cycle to define a minimum overtravel distance `a` determined by lost motion connection means 316 (which may be considered the first lost connection means). Only after the plunger sections 308 and 310 are separated by the overtravel distance `a` will first extension spring 318 begin to apply its preload tension to bias the plunger sections together. In a manner similar to the operation of the embodiment of FIG. 1, the second lost motion connection means 320 prevents second extension spring 322 from applying any additional biasing force to the plunger sections until the sections are separated by an additional "advance" distance `b` determined by the characteristics of the second lost motion connection means 320.

First lost motion connection means 316 includes a pin 316a and anchor element 316b similar to the pin and anchor element of the embodiment of FIG. 1 except that anchor element 316b is directly connected by screw threads to the outer end of first spring 318 rather than the second spring 322. The second lost motion connection means 320 includes a second pin 320a and second anchor element 320b positioned within a downwardly opening recess 316c contained in anchor element 316b. Pin 320a is fixed to anchor element 320b and the outer ends thereof are held within longitudinally oriented slots 320c contained in anchor element 316b. Thus, the axial length of slots 320c define the amount of lost motion which may occur between anchor elements 316b and 320b before second spring 322 begins to apply a tension force which tends to pull the plunger sections together.

The functional benefits which derive from provision of the second lost motion connection means 320 can best be understood by reference to FIG. 3a in which the travel profile of outer (upper) plunger section 310 is plotted against degrees of cam rotation when the injector is operating in the retard mode, that is when the timing fluid pressure is insufficient to overcome the preload tension on spring 318 and thus the plunger sections are only separated by the distance `a` defined by lost motion connection means 316. The sections become separated by the distance `a` because a predetermined minimal pressure is always provided by the timing fluid. In response to this minimal pressure, the timing chamber 307 formed between sections 308 and 310 will always expand as the outer plunger section 310 moves outwardly. This minimal fluid timing pressure will not, however, overcome the preload tension in spring 318. Upon inner moement of the injector plunger sections by a cam (profiled, for example, like the cam of FIG. 1), the inner plunger section 308 will naturally reach the cup 324 of the injector body 302 before the outer plunger section 310 has reached its innermot position. This fact is shown by point i₁ in FIG. 3a, which shows that the outer plunger section 310 is still traveling in an inner direction when the inner plunger section 308 reaches the injector cup 324 to close off the injection orifice. At this time, the timing fluid trapped in the expanded timing chamber 307 is expelled through drain passage 312 at very high pressure to maintain the inner plunger section 308 in its innermost position. This mode of operation contrasts with the retard mode (non-advanced mode) of operation of the FIG. 1 embodiment in which the variable volume chamber 230 is fully collapsed before the forward plunger section 244 reaches the injector cup. This is illustrated by point i₂ in FIG. 3a. Because point i₁ occurs along a higher velocity portion of the travel profile, the inner plunger section 308 engages the injector cup 324 at a higher velocity than does the inner plunger section 224 of FIG. 1 to produce thereby a sharper end of fuel flow during injection. Moreover, the need for dimple 284 on the cam profile no longer exists since the timing fluid within the timing chamber insures that the inner plunger section 308 will be held tightly against the injector cup during the final stages of inward movement of the outer plunger section 310. FIG. 3a shows only a minimal overtravel `d` of 0.018 inches after the plunger sections are brought into contact but before the outer plunger 310 reaches its innermost position. This arrangement allows elimination of the dimple 284 of the FIG. 1 cam profile which would have resulted in the additional overtravel of outer plunger 310 as illustrated by the dashed lines in FIG. 3a. Naturally, cam life is increased when the "crush load" created by dimple 284 can be eliminated. An additional benefit of the embodiment of FIG. 3 is that any wear which takes place in the injector actuation train is automatically compensated for by the creation of a minimal hydraulic link during the retard mode of injector operation.

Yet another embodiment of the subject invention is illustrated in FIG. 4 wherein a unit injector 400 is illustrated including an injector body 402 having three axially spaced recesses 404, 406 and 408 formed on the exterior thereof in axially spaced relationship. The innermost recess 404 serves to form a fluid supply path with the recess in which the injector is adapted to be placed. The middle recess 406 is adapted to form a fuel drain flow path communicating with a drain port (not illustrated) opening into a recess within which the injector is adapted to be placed. Finally, outer recess 408 forms a timing fluid flow path communicating with a timing port (not illustrated) which also opens into the recess within which the injector 400 is adapted to be placed. Thus, it can be seen that the timing channel and drain channel for use in combination with the embodiment of FIG. 4 has been reversed from the position illustrated in FIGS. 1 and 3.

The injector plunger 412 is formed in two sections including an inner plunger section 414 and an outer plunger section 416 defining a variable volume timing chamber 426 between them. The plunger sections 414 and 416 are biased together by a biasing means including first and second coaxial extension springs and a lost motion connection means identical to that used in the embodiment of FIG. 1.

On the exterior of inner plunger section 414, an annular groove 418 is formed to communicate with supply passage 420 when the inner plunger section 414 is in its innermost position as illustrated in FIG. 4. A drain passage 422 is formed to communicate with the fuel drain flow path formed by recess 406. Both the supply passage 420 and drain passage 422 are perpendicular to the longitudinal axis of the injector and thus are very easily manufactured. As constructed in the manner illustrated in FIG. 4, the unit injector 400 operates with regard to fuel metering and injection in a manner identical to that of the FIG. 1 embodiment of this invention. For purposes of advancing timing, however, timing fluid connecting means 424 is provided for forming a pathway for timing fluid to flow from the timing fluid flow path formed by recess 408 into the variable volume timing chamber 426 formed between inner and outer plunger sections 414 and 416. The timing fluid connecting means 424 includes a timing passage 425 extending from the exterior surface of the injector body to the interior surface of the central bore along a straight radial path perpendicular to the longitudinal axis of the injector body 402. Again, by this arrangement, only a minimal manufacturing expense is involved in forming the timing passage.

Scavenging takes place when the plunger 412 is in its innermost position as illustrated in FIG. 4 to allow fuel to flow through supply passage 420, annular groove 418 and drain passage 422. Scavenging thus takes place in the same manner as described in the co-pending, commonly assigned, application of Les Peters entitled "Fuel Injector Assembly Including Blow-Back Prevention Cam".

As is apparent from FIG. 4, the variable volume timing chamber 426 is fully collapsed. However, when the injector plunger 412 is in its outermost position, the variable volume chamber 426 comes in communication with the timing passage 425. In the time that the inner injector plunger 416 is held in its outermost position, timing fluid may pass into the variable volume timing chamber 426. In the embodiment of FIG. 4, the amount of fluid which flows into the timing chamber 426 is a function solely of the lost motion connection distance defined by the lost motion connection means 440 which operates in the same manner as the lost motion connection means 262 of FIG. 1. When the inner plunger section 414 is moved to its innermost position, the variable volume timing chamber 426 will be positioned to communicate with a timing fluid discharging means 428 for providing a pathway for timing fluid to flow from the timing chamber 426 into the drain flow path formed by recess 406. By this arrangement, the timing chamber collapses completely during each cycle of the injector. Means 428 includes a discharge passage 429 which is shaped like the drain passage 312 of FIG. 3. Thus, in the embodiment of FIG. 4, the function performed by drain connecting means 264 of FIG. 1 (i.e., spilling timing fluid and discharging scavenging fuel flow) requires both discharge passage 429 and drain passages 422.

To provide the braking mode fuel cut off function referred to with respect to the embodiment of FIG. 1, the injector body 402 includes a fuel cut off passage 430 extending obliquely from the exterior surface of the injector body 402 defined by recess 408 to the central bore of the injector body 402 at an axial position which allows flow of timing fluid into the timing chamber 426 when the inner plunger section 414 is in its innermost position. Thus, upon increase of the timing fluid pressure by an amount which allows the force within chamber 426 to exceed the combined force applied by the pair of extension springs forming the plunger biasing means referred to above, the inner plunger section 414 will be held in its innermost position while the outer plunger section 416 will reciprocate under the control of the cam actuating mechanism (not illustrated).

A check valve means 432 is provided in the fuel cut off passage 430 for permitting timing fluid to flow only one way through the fuel cut off passage 430 into the timing chamber 426. Check valve means 432 insures that a hydraulic link is maintained at all times between the injector plunger sections 414 and 416 during the inward stroke of the plunger until the inner plunger section 414 reaches its innermost position.

To insure against possible leakage of fuel which ma seep upwardly between the injector plunger and a central bore of the injector body 402, a leakage connecting means 434 is provided for forming a pathway for leaked fuel to flow into the drain flow path formed by recess 406. The leakage connecting means 434 includes an annular cavity 436 extending outwardly from the central bore at a point above the uppermost limit reached by the variable volume timing chamber 426 and a leakage passage 438 extending from the annular cavity to the drain pathway formed by recess 406.

It should now be clear that the embodiment of FIG. 4 operates in a manner similar to the embodiment of FIG. 1 with respect to fuel metering and with regard to the timing chamber except that timing fluid is provided to the variable timing chamber 426 only upon the outer plunger section 416 reaching the axial position of passage 425 whereas in the embodiment of FIG. 1 timing fluid is supplied at all times to the variable volume chamber.

Reference is now made to still another embodiment of the unit injector illustrated in FIGS. 5 and 6. FIG. 5 illustrates a unit injector 500 having an injector body 502 similar to the injector body 402 illustrated in FIG. 4. One main difference is that the fuel cut off passage has been eliminated and the drain passage 503 has been moved to the opposite side of the injector from the timing fluid discharge passage 505 and the timing passage 507. Like the embodiment of FIG. 4, passages 503 and 505 together form a drain connecting means for removing fuel from the central bore of the injector body 402.

The biasing means in the embodiment of FIGS. 5 and 6 employs a pair of compression springs 504a and 504b rather than the extension springs of embodiments 1 and 3. In particular, the outer plunger section 506 is formed by a hollow sleeve 506a to define an interior cavity 506c. The inner end of sleeve 506a includes an inwardly directed tip 508. In addition to the compression springs 504a and 504b, the biasing means includes a connecting pin 512 having a shank portion 514 connected at one end to the inner plunger section 510 and extending into the interior cavity 506c of the outer plunger section 506. The connecting pin 512 further includes an enlarged head 516 at the end remote from the inner plunger section 510. The inner compression spring 504b is positioned between lip 508 and enlarged head 516. In this embodiment, connection pin 512 forms part of the inner plunger section 510 such that spring 504b can be considered to be extending "between" the inner and outer plunger sections. Compression spring 504b operates at all times to apply a force tending to move plunger sections 506 and 510 together. Compression spring 504a is co-axial with spring 504b and extends from lip 508 toward the lower edge of plug-like element 506b. A ring element 504c separates the outer end of compression 504a from the inner end of plug-like element 506b. Because ring element 504c extends radially inwardly, head 516 will engage ring element 504c whenever the inner and outer plunger sections 506 and 510 are separated by more than a predetermined distance. Only when the plunger sections are so separated will compression spring 504a become effective to add its biasing force to that of compression spring 504b to bias the plunger section toward one another. Accordingly, dual compression springs 504a and 504b will function in a manner similar to the dual extension spring and lost motion connection means of the embodiment of FIG. 1.

The operation of the unit injector illustrated in FIGS. 5 and 6 should be apparent from the drawings and from the prior description of the operation of the injectors illustrated in FIGS. 1 and 4. In particular, FIG. 5 illustrates on the left hand side of the center line the position of the inner and outer plunger sections when the timing chamber is fully collapsed and the plunger is in its innermost position. On the right hand side of the center line of FIG. 5, the plunger sections are shown in their outermost position and the timing function is about to begin. Timing fluid will flow into the timing chamber only if the timing fluid pressure is sufficient to overcome the biasing force of spring 504b. On the left, the position of the plunger sections is shown when the timing chamber has been expanded. Still further as a result of the timing fluid pressure being sufficient to overcome the bias of both compression springs 504a and 504b. When the plungers assume this position fuel metering will be cut off.

The graph of FIG. 7 clarifies the operational differences between the single lost motion/dual spring embodiment of FIGS. 1, 4, 5 and 6 as compared with the dual lost motion/dual spring embodiment of FIG. 3. In particular, FIG. 7 is a graph of plunger section separation versus timing supply pressure. Line 1 represents the relationship of plunger separation versus timing supply pressure for the embodiment of FIG. 3 wherein the plunger sections will separate during each injector cycle as illustrated by line segment 1₁. This minimum separation insures a hydraulically imposed "rush load to produce a sharper end of each injector due to the overtravel function of the plunger." Line 1₂ illustrates the relationship of plunger separation versus timing pressure of the remaining embodiment (FIGS. 1, 4, 5 and 6) which embodiments do not provide overtravel in the retard mode. This relationship is shown by line segment 1₂ in FIG. 7 although no specifically shown in any of the embodiments described above, it would be possible to employ a single lost motion connection and a single spring element arranged to provide a bias tending to bring the plunger sections together only after the plunger sections are separated by the distance defined by the lost motion connection. By such an arrangement, the overtravel feature could be provided to an injector without a fuel cut off capability.

As is apparent from the foregoing, the timing fluid supplied to any of the variable timing embodiments discussed in this disclosure would normally be engine fuel since this would eliminate any problems associated with mixing of dissimilar fluids in the engine drain channel.

    ______________________________________                                         Index to Reference Numbers                                                     ______________________________________                                         200       unit injector                                                        202       injector body                                                        202a      barrel or outer section                                              202b      cup or inner section                                                 202c      screw threads                                                        203       recess                                                               203a      hexagonal socket                                                     204       supply channel                                                       205       head                                                                 206       drain channel                                                        208       supply port                                                          210       drain port                                                           212       timing channel                                                       214       timing port                                                          216       supply flow path                                                     217       supply connecting means                                              218       supply passage                                                       219       freed orifice                                                        222       injector plunger                                                     223       forward end                                                          224       inner plunger section                                                226       outer plunger section                                                226a      inwardly opening recess                                              228       central bore                                                         230       variable volume timing chamber                                       232       timing fluid flow path                                               234       annular recess (timing)                                              236       annular recess (supply)                                              238       annular recess (drain)                                               240       drain flow path                                                      241       circumferential seal                                                 241a      small annular groove                                                 242       timing fluid connecting means                                        243       circumferential seal                                                 243a      small annular groove                                                 244       timing passage                                                       245       washer-like element                                                  246       flow control means                                                   247       inwardly directed lip                                                248       annular groove                                                       248a      cylindrical land                                                     249       outwardly directed surface                                           250       transverse passage                                                   251       injection orifices                                                   252       recess                                                               253       outer circumferential seal                                           253a      small annular groove                                                 254       ball type check valve                                                256       biasing means                                                        258       first helical spring                                                 258a      screw threads                                                        258b      screw threads                                                        260       second helical spring                                                260a      screw threads                                                        260b      screw threads                                                        262       lost motion connection means                                         262a      pin                                                                  262b      anchor element                                                       262c      elongated opening                                                    264       drain connecting means                                               265       leakage connecting means                                             266       drain passage                                                        266a      restricted portion                                                   268       radial passages                                                      269       leakage passage                                                      270       cam actuated mechanism                                               270a      arrows                                                               271       cam lobe                                                             272       push rod                                                             272e      arrows                                                               273       rocker arm                                                           274       link                                                                 274e      arrow                                                                275       plunger return means                                                 276       sleeve like coupler                                                  277       lower end                                                            278       outwardly directed flange                                            279       compression spring                                                   280       one sector                                                           281       second sector                                                        282       third sector                                                         283       fourth sector                                                        284       raised dimple                                                        300       injector unit                                                        302       injector body                                                        304       supply passage                                                       306       timing passage                                                       307       variable length timing chamber                                       308       inner plunger section                                                310       outer plunger section                                                312       dual function drain and discharge passage                            314       leakage passage                                                      316       first lost motion connection means                                   316a      pin                                                                  316b      anchor element                                                       318       first extension spring                                               320       second lost motion connection means                                  320a      pin                                                                  320b      anchor element                                                       320c      slots                                                                322       second extension spring                                              324       injector cup                                                         400       unit injector                                                        402       injector body                                                        404       innermost recess                                                     406       middle recess                                                        408       outer recess                                                         412       injector plunger                                                     414       inner plunger section                                                416       outer plunger section                                                418       annular groove                                                       420       supply passage                                                       422       drain passage                                                        424       timing fluid connecting means                                        425       timing passage                                                       426       timing chamber                                                       428       timing fluid discharging means                                       429       discharge passage                                                    430       fuel cut off passage                                                 432       check valve means                                                    434       leakage connecting means                                             436       annular cavity                                                       438       leakage passage                                                      440       lost motion connection means                                         500       unit injector                                                        502       injector body                                                        503       drain passage                                                        504a      compression spring                                                   504b      compression spring                                                   504c      ring                                                                 505       timing fluid discharge passage                                       506       outer plunger section                                                506a      hollow sleeve                                                        506b      plug-like element                                                    506c      interior cavity                                                      507       timing passage                                                       508       inwardly directed tip                                                510       inner plunger section                                                512       connecting pin                                                       514       shank portion                                                        516       enlarged head                                                        ______________________________________                                    

Industrial Applicability

A miniaturized unit injector designed in accordance with this invention would find application in a very large variety of internal combustion engines. One particularly important application would be for small compression ignition engines adapted for powering automobiles. Lighter truck engines and medium range horsepower engines could also benefit from the use of injectors designed in accordance with the subject invention. 

I claim:
 1. A fluid injector, comprising(a) an injector body containing a central bore and an injection orifice communicating with said central bore; (b) an injector plunger mounted for periodic reciprocal movement within said central bore between an innermost position in which said injection orifice is closed and an outermost position in which an injection chamber is formed within said central bore between the inner end of said injector plunger and said injection orifice, said injector plunger including an inner plunger section and an outer plunger section mounted for independent reciprocal movement within said central bore to define a variable volume timing chamber between said plunger sections; (c) supply connecting means for providing a pathway for fluid under pressure to flow from a source into said injection chamber; (d) timing fluid connecting means for providing a pathway for fluid under pressure to flow from a timing fluid source into said timing chamber; and (e) biasing means for biasing said first and second plunger sections together with a force which is a stepped function of the axial distance between said plunger sections.
 2. An injector as defined in claim 1, wherein said biasing means includes a pair of oppositely wound helical extension springs and wherein said plunger sections include complementary screw threads for engaging the ends of said extension springs.
 3. An injector as defined in claim 2, wherein said biasing means includes a first extension spring connected integrally at each end, respectively, to said plunger sections to apply at all times a first predetermined force tending to bring said plunger sections together, and further including a second extension spring connected integrally to one of said plunger sections, and a lost motion connection means for connecting the other end of said second extension spring to the remaining said plunger section to cause said second extension spring to apply a second additional predetermined force tending to bring said plunger sections together whenever said plunger sections are separated by a first predetermined distance.
 4. An injector as defined in claim 1, wherein said biasing means includes a first extension spring connected at each end, respectively, to said plunger sections to apply a first predetermined force tending to bring said plunger sections together, a second extension spring connected at each end to said plunger sections, and a lost motion connection means for providing lost motion between the end of said second extension spring and one of said plunger sections to cause said second extension spring to apply a second predetermined force tending to bring said plunger sections together whenever said first and second plunger sections are separated by more than a first predetermined distance.
 5. An injector as defined in claim 4, wherein said biasing means includes a second lost motion connection means to define a lost motion between the end of said first spring and said first mentioned lost motion connection means, whereby no biasing force is applied to said plunger sections by said extension springs until said plunger sections are separated by a first predetermined distance defined by said first mentioned lost mostion connection means and whereby only said first mentioned extension spring applies a biasing force to said plunger sections when said plunger sections are separated by more than said first predetermined distance but less than a second predetermined distance equal to the sum of the lost motions defined by said first and second lost motion connection means and further whereby both said extension springs apply a biasing force to said plunger sections when said plunger sections are separated by more than the sum of the lost motion defined by said first and second lost motion connection means.
 6. An injector as defined in claim 4 for use with a source of timing fluid which may be varied between a timing advance pressure and a fuel cut off pressure which is greater than said timing advance pressure, wherein the spring rates of said first and second extension springs and the effective cross sectional areas of said plunger sections are chosen such that(a) the plunger section separating force created by supplying timing fluid at said timing advance pressure to said timing chamber is greater than said first predetermined force but is less than the sum of said first and second predetermined forces and (b) the plunger section separating force created by supplying timing fluid at said fuel cut off pressure level to said timing chamber is greater than the sum of said first and second predetermined forces.
 7. An injector as defined in claim 5 fur use with a source of timing fluid which may be varied among a residual pressure, a timing advance pressure which is greater than the residual pressure and a fuel cut off pressure which is greater than the timing advance pressure, wherein the spring rates of said first and second extension springs and the effective cross sectional areas of said plunger sections are chosen such that(a) the plunger section separating force created by supplying timing fluid at said residual pressure level to said timing chamber is less than said first predetermined force, (b) the plunger section separating force created by supplying timing fluid at said timing advance pressure to said timing chamber is greater than said first predetermined force but is less than the sum of said first and second predetermined forces, and (c) the plunger section separating force created by supplying timing fluid at said fuel cut off pressure level to said timing chamber is greater than the sum of said first and second predetermined forces.
 8. A fuel injector for use in an injector receiving recess of an internal combustion engine for injecting periodically into a combustion chamber of the engine a portion of the fuel received from a supply port opening into the recess and for returning all the fuel received and not injected to an axially spaced drain port which also opens into the recess and further having a timing port opening into the injector receiving recess for supplying variable pressure timing fluid to the fuel injector, said fuel injector comprising(a) an injector body shaped to be inserted into the recess to form a supply flow path extending between the supply port and the exterior surface of said injector body and to form further a drain flow path, isolated from said supply flow path, extending between the drain port and the exterior surface of said injector body and to form still further a timing flow path, isolated from said supply and drain flow paths, extending between the timing port and the exterior surface of said injector body, said injector body including a central bore coaxial with the longitudinal axis of said injector body and an injection orifice located adjacent the inner end of said central bore; (b) supply connecting means providing a pathway for fuel under pressure to flow from said supply flow path into said central bore; (c) drain connecting means providing a pathway for fuel to flow from the central bore into the drain flow path; (d) an injector plunger mounted for reciprocal movement within said central bore between an innermost position in which said injection orifice is closed by the inner end of said injection plunger and an outermost position in which an injection chamber is formed within said central bore between said inner end of said plunger and said injection orifice, said injector plunger includes(1) an inner plunger section and an outer plunger section mounted for independent reciprocal movement within said central bore to define a variable volume timing chamber between said plunger sections, and (2) biasing means for biasing said inner and outer plunger sections together with a force which is a stepped function of the axial distance between said plunger sections; and (e) timing fluid connecting means providing a pathway for fluid flow from said timing flow path into said central bore to form selectively a hydraulic link between said plunger sections.
 9. An injector as defined in claim 1, further including a drain connecting means providing a pathway for fluid to flow from said central bore to the exterior of said injector body.
 10. An injector as defined in claim 8 or 9, wherein said drain connecting means includes a discharge passage for timing fluid to flow from said timing chamber to the exterior of said injector body for a sufficient time during each reciprocal movement of said injector plunger to cause said timing chamber to collapse, said discharge passage including a restriction to cause sufficiently high fluid pressure to develop within said timing chamber during collapse of said timing chamber to hold said inner injector plunger in its innermost position.
 11. An injector as defined in claim 8, wherein said injector body includes three axially spaced recesses on the exterior surface shaped to form the supply, drain and timing flow paths, respectively, and further including seal means for forming circumferential seals between said injector body and the recess within which said injector body is designed to be operatively placed in order to isolate fluidically the supply, drain and timing flow paths.
 12. An injector as defined in claim 9, wherein said supply connecting means includes a supply passage extending from said central bore to said annular recess forming the supply flow path, wherein said drain connecting means includes a drain passage extending between said central bore and said annular recess forming the drain flow path and said timing fluid connecting means includes a timing passage extending between said central bore and said annular recess forming the timing fluid flow path.
 13. An injector as defined in claim 12, wherein said supply passage and said timing passage are formed along respective straight paths perpendicular to the longitudinal axis of said injector body.
 14. An injector as defined in claim 12, wherein said drain passage is formed along a straight path which is perpendicular to the longitudinal axis of said injector body.
 15. An injector as defined in claim 1 or 8, wherein said injector body includes a fuel cut off passage extending from the exterior surface of said injector body to said central bore at an axial position which allows flow of timing fluid into said timing chamber when said injector plunger is in its innermost position, and a check valve means for permitting only one way flow through said fuel cut off passage into said timing chamber.
 16. An injector as defined in claims 1 or 8, wherein said biasing means includes a pair of compression springs.
 17. An injector or as defined in claims 1 or 8, wherein said biasing means includes at least one spring extending between said plunger sections and a lost motion connection means which prevents said spring from imposing a biasing force to mid plunger sections until said plunger sections are separated by more than a predetermined distance. 